A pioneer was anyone who was the first to settle in a region. They were known for their adventurous spirit. (TNG: "Silicon Avatar") As such, it was the name of several space probes and a baseball team.
James T. Kirk's ancestors were American pioneers. When Spock suggested that the Melkot chose the battle at the OK Corral for the USS Enterprise landing party's punishment based upon Kirk's cultural past, Kirk agreed. (TOS: "Spectre of the Gun")
Alternately, it could also refer to a scientist who was one of the first in their field. Sarah April was known for being a pioneer doctor in space. (TAS: "The Counter-Clock Incident") Rose Kumagawa, John Kelly, Neil Armstrong, and John Glenn were also considered space exploration pioneers. (VOY: "One Small Step")